The presence of ice on a surface can be detected by electrical, acoustical, mechanical and optical devices. Most of these devices are used to determine the presence of or characteristics of ice on aircraft. In a report No. DOT/FAA/CT-92/27 entitled "Aircraft Ice Detectors and Related Technologies for Onground and Inflight Applications", published in 1993 and authored by Gregory A. Hoover, Galaxy Scientific Corporation, for the U.S. Department of Transportation, Federal Aviation Administration, Technical Centre, Hoover describes several existing technologies for ice detection.
In U.S. Pat. No. 5,243,185 entitled Apparatus and Method for Ice Detection, in the name of Carl Blackwood, various patents related ice detection are described. For example, U.S. Pat. No. 4,701,052 in the name of Schoen discloses a dew point hygrometer that uses linearly-polarized fight to detect condensate on a metal surface. Schoen declares the presence of condensation when a light detector receives on light form an analyzer.
Another U.S. Pat. No. 4,725,145 in the name of Azzam is described as disclosing a polarimeter from which one parameter of polarization of light beam can be determined. The Azzam invention is a polarization sensitive photodetector that produces an output proportional to the radiation absorbed by the photodetector.
U.S. Pat. No. 4,668,860 (Anthon) discloses a scattermeter for evaluating the surface quality of an optical element, wherein surface scatter is distinguished form bulk scatter by differing polarization characteristics. Neither Azzam nor Anthon incorporate the intrinsic birefringent properties of the hexagonal crystalline structure of the ice crystal.
Blackwood, in U.S. Pat. No. 5,243,185, on the other hand uses the birefringent properties of the ice crystal to determine its presence on a polarization preserving surface. Blackwood discloses an apparatus and method that analyzes elliptically-polarized reflection of linearly-polarized radiation from an ice-covered surface. A linearly-polarized radiation beam (15) is focused on a target surface (20), and returned radiation (25) is filtered (30) as a function of its elliptical polarization. Filtered radiation (35) is directed to a sensor (40) and quantified according to the intensity of radiation received at various points (41.sub.1 -41.sub.n) on the filter (30), and an output signal for each point is produced (45.sub.1 -45.sub.n). The presence of ice is detected by a signal processor that detects a variance among the output signals. Although Blackwood's invention appears to adequately perform its intended function of detecting as polycrystalline structure on a polarization preserving surface, it provides a complex solution to the problem of detecting ice. Furthermore, Blackwood's invention is not related to detecting a single crystalline layer of ice on a target surface.
It is an object of this invention to provide a method and device for simply, remotely detecting the presence of birefringent material on a polarization preserving surface.
It is a further object of this invention to provide an relatively inexpensive system to manufacture, for remote detection of ice.